Concentrations of heavy metals in soils were generally observed to be higher during the wet season in both locations although the differences were not statistically significant (P > 0.05). This differs somewhat with the findings of (Najib et al., 2012), who observed a higher concentration of these metals in the dry season than the wet season. Seasonal variations in patterns of metal deposition could be related to the intensity and duration of climatic variables such as precipitation, temperature etc., that interface with topography, drainage, soil structure/texture etc., to determine the physicochemical properties of the soil in a particular location. Soil physicochemical properties have complex, interdependent effects on metal solubility, with the most important of these including solution composition (inorganic and organic solubles), Eh, and pH; type and density of charge on soil colloids; and reactive surface area, that interact with factors like metal concentration and form, particle size distribution, quantity and reactivity of hydrous oxides, mineralogy, degree of aeration and microbial activity (Cataldo and Widung 1987). The aggregate effects of these complex interactions determine the bioavailabilty of metals to plants.
The above acceptable limits of the observed concentrations of Cr, Cu, Ni and Zn may be attributed to some of the human activities (mainly agriculture and industrial) going on around the sites. This presents health risks to humans and other animals as the metals contaminate both aquatic and terrestrial ecosystems. Above background values of these metals may have resulted from metal smelting and electroplating activities, burning of fossil fuels, application of phosphate fertilizers, disposal of solid wastes, and quarry activities (Kabata-Pendias and Pendias, 1992; Knox et al., 1999; Alloway, 1995; Liu et al., 2005). High levels of heavy metals in terrestrial or aquatic ecosystems ultimately end up being transmitted and accumulated in the food chain. Health risks to humans arise when metal polluted water is used as drinking water or when animals that have consumed vegetative materials in which metals have accumulated, are used for food. Furthermore, cultivation of crops on metal polluted soils indicates a possibility of consuming crops in which metals have accumulated. Although, specific effects of the various metals on human health have been discussed by several authors, the biotoxic effects of metals to humans have generally been outlined as ranging from gastrointestinal disorders, diarrhoea, stomatitis, tremor, hemoglobinuria, ataxia, paralysis, vomiting and convulsion, depression, coughing and wheezing, respiratory inflammation, dermatitis, leukocytis, low blood pressure, jaundice haemolytic anemia pneumonia and coma to death (Cd, Pb, As, Hg, Zn, Cu, Cr, Ni and Al). The nature of these effects could be toxic (acute, chronic or sub-chronic), neurotoxic, carcinogenic, mutagenic or teratogenic. For example, see Duruibe et al. (2007);Bako et al. (2009a); Bako et al. (2009b); and Friends of Nature (2010).
Concentrations of Cd, Zn, Cr, Cu and Ni in all the plants observed were found to be below levels described as phytotoxic to plants. However, above ‘normal’ concentrations of Cr was observed in most of the plant species sampled (Galfata et al., 2013). This may be a consequence of the high values observed in the soils or direct deposition from the atmosphere (Jankowski et al. 2019). In addition to soil concentrations of metals, other factors that determine the uptake, translocation and accumulation of metals in plants include soil pH, cation exchange capacity, organic matter content, soil texture and interaction with other metals, as well as translocation factor (rate of movement of metals between root and shoot tissues) for the particular metal (Yoon et al., 2006; Baker and Brooks, 1989; Jung, 2010; Nwachukwu and Agbede, 2010; Guala et al., 2010, Addo et al., 2010). Heavy metals in toxic concentrations have inhibitory effects on enzymatic activity, stomatal function, photosynthesis and nutrient uptake, which may be expressed visually as chlorosis, reduced/stunted growth and yield depression. Plants vary widely in their ability to tolerate high concentrations of metals in their tissues. This variation is usually natural and dependent on inherent genetic factors. The genetic disposition confers the ability to employ a range of avoidance/exclusion or detoxification mechanisms that enable the plants cope with high metal loads. These may include the binding of metals (eg Ni and Cr) with amino acids, peptides and organic acids to form low molecular weight compounds, formation of phytochelatins, by binding (e.g. Cu and Pb) with sulphur-rich proteins and cellular adaptations. Other strategies may involve roles for mychorrhiza, the cell wall, extra-cellular exudates, efflux pumping mechanisms in the plasma membrane and formation of stress proteins etc (Bako et al., 2005; Kotrba et al., 2009).
Plants with BCF of metals > 1.0, have been described as suitable for phytoextraction. Some of the plants observed in this study with this potential include; Terminalia macroptera, Centrosema pubescens, Piliostigma thonningii, Cochlospermum tinctorum, Isoberlinia tomentosa, Monotes kerstingii, Detarium microcarpum, Paspalum orbiculare, Borreria verticelata, Salix leadermanii, Ceruana pratensis, Polygonum lanigerum, Physalis angulata, Polygonum limbatum, Cymbopogon giganteum, Heliotropium indicum, Croton lobatus and Mimosa pigra for Cu; and Centrosema pubescens, Hypoethes cancellata and Polygonum lanigerum for Cd. The ability of these plants to concentrate high levels of these metals suggests that they may have a good potential for phytoremediation.
No hyper-accumulator was observed in this study. Hyper-accumulators are plants that can accumulate at least 0.1% wt of Cu, Cd, Cr, Pb, Ni and Co or 1% wt of Zn and Mn (Baker and Brooks, 1989). There are possibilities for genetic modification of plants to enhance their capacity for metal tolerance (Kotrba et al., 2009).
RECOMMENDATIONS FOR FURTHER RESEARCH
There is a great need to establish environmentally safe limits for metals in plant and soils of the various eco-regions in Nigeria. This need is emphasized by the observed variations in published background values from one country to another and even within the same country. These background values are often dependent on the geological history of the area. A comparison of observed field values with national recommended levels for heavy metals, developed from the background values will give a more realistic assessment of the pollution status. Furthermore, the search for alternative green technology that can be employed in remediation of pollution events must necessarily be a continuous one, due to the relative low cost and environmental friendliness of this option as compared to others. In this regard, ruderal species rather than those with agricultural value must be the candidates of choice for avoidance of obvious conflicts.